The present invention generally relates to vehicle suspensions. More particularly, the present invention relates to vehicle air suspension systems that include a strut assembly comprising an air cylinder, coil spring, and damper, which may be referred to as cylinder shock assembly or strut assembly. Prior strut assemblies have included a flexible air spring wherein the air spring includes a flexible membrane that expands as the strut is loaded. Typical air springs of this type with a flexible member provide a relatively low spring rate when compared to a mechanical spring. As a result, additional spring rate and roll stiffness has typically been required to be added to the vehicle, e.g., often in the form of an additional, independent mechanical coil spring to meet vehicle handling and stability objectives. Therefore, when using air springs with flexible or expandable membranes, an ‘anti-roll bar’ or ‘sway bar’ has often been utilized as the mechanism to achieve the desired auxiliary roll stiffness.
In typical air springs with a flexible membrane, the operating pressures that may be utilized are limited because of the current limitations on flexible membrane and rubber construction. Thus, the typical operating pressure at normalized ride height is limited to 80-100 psi, up to 120 psi, with maximum pressure at full compression at around 200 psi. It is known that by increasing the effective spring rate of the strut assembly, the vehicle may achieve a higher ride frequency, and increased roll performance and lateral stability performance. In particular, when the effective spring rate of the strut at the designed ride height is increased, the need for auxiliary anti-roll devices may be eliminated.
In some applications using an air spring with a flexible membrane, a coil spring may be added to increase the effective spring rate of the strut. However, to achieve a desired effective spring rate at the designed ride height, the strut may require a larger spring, a larger volume of air (and thus a flexible membrane with a larger diameter) or a combination of both. In many applications, however, the space constraints, or limited footprint available for a strut, do not allow for an air spring with a flexible membrane even when combined with a coil spring to achieve the desired increased effective spring rate at the designed ride height of a vehicle because of the increased size of the strut required to achieve the desired effective spring rate.
Therefore, in some applications it would be desirable to provide a strut capable of operating at increased operating pressures, to achieve a desired increased effective spring rate at the designed ride height that fits within existing space constraints. In addition, it would be desirable to provide a strut having an increased effective spring rate. For example, in some applications it may be desirable to provide a strut having an effective spring rate high enough so that the need for auxiliary roll resisting devices such as an anti-roll bar is not required to achieve the desired vehicle roll resistance and lateral stability.
In addition, typical air springs with a flexible membrane that also include a coil spring include a bump stop that is engaged once the coil spring is compressed to a certain point, which results in an abrupt change in spring rate which in turn causes an undesirable jolt to the vehicle's sprung mass. Accordingly it would be desirable to provide a strut that provides a continuously increasing spring rate without any abrupt changes in spring rate or spring rate slope to provide for smoother vehicle and passenger ride performance.